In a solar cell, a polycrystalline or single crystal Si cell is used as a semiconductor substrate.
A configuration of a conventional solar cell will be described based on a solar cell 50 of the present invention shown in FIGS. 4A and 4B. The solar cell 50 is manufactured by bonding solar cell lead wires 10a and 10b to a predetermined region of a semiconductor substrate 52, i.e., to a front surface electrode 54 provided on a front surface of the semiconductor substrate 52 and to a back surface electrode 55 provided on a back surface thereof, using a solder. Electricity generated in the semiconductor substrate 52 is transmitted to the outside through the solar cell lead wire.
A configuration of a conventional solar cell lead wire will be described based on a solar cell lead wire 10 of the present invention shown in FIGS. 1A and 1B. A solar cell lead wire 10 is provided with a strip-shaped conductive material 12 and a molten solder plated layer 13 formed on upper and lower surfaces of the strip-shaped conductive material 12. The strip-shaped conductive material 12 is, e.g., a circular cross-section conductor roll-processed into a strip shape, which is called a flat conductor or a flat wire.
The molten solder plated layer 13 is formed by supplying a molten solder on the upper and lower surfaces of the strip-shaped conductive material 12 using a hot-dip coating method.
The hot-dip coating method is a method in which the upper and lower surfaces of the strip-shaped conductive material 12 are cleaned by acid pickling, etc., and a solder is laminated on the upper and lower surfaces 12a and 12b of the strip-shaped conductive material 12 by passing the strip-shaped conductive material 12 through a molten solder bath. As shown in FIG. 1A, the molten solder plated layer 13 is formed in a shape bulging from a side portion in a width direction to a center portion, so-called a mountain-like shape, by an effect of surface tension at the time of solidification of the molten solder adhered on the upper and lower surfaces 12a and 12b of the strip-shaped conductive material 12.
The solar cell lead wire 10 is cut to a predetermined length, is sucked up by air suction and moved onto a front surface electrode (grid) 54 of the semiconductor substrate 52, and is soldered to the front surface electrode 54 of the semiconductor substrate 52. An electrode band (finger) (not shown) electrically conducting with the front surface electrode 54 is preliminarily formed on the front surface electrode 54. The molten solder plated layer 13 of the solar cell lead wire 10a is brought in contact with the front surface electrode 54, and soldering is carried out in this state. The soldering of the solar cell lead wire 10b to the back surface electrode 55 of the semiconductor substrate 52 is carried out in the same way.
Conventionally, the front surface electrode 54 is impregnated with solder of the same nature as the molten solder plated layer 13 of the solar cell lead wire 10 in order to impart good solder bondability (or soldering strength) between the front surface electrode 54 of the semiconductor substrate 52 and the solar cell lead wire 10. However, the semiconductor substrate 52 has become thinner in recent years and a problem of damage to the semiconductor substrate 52 at the time of impregnating the front surface electrode 54 with the solder has emerged. Therefore, omission of solder impregnation process performed on the front surface electrode 54 has been promoted in order to avoid damage to the semiconductor substrate 52.
Due to the omission of solder impregnation process which is performed to impart good solder bondability between the front surface electrode 54 of the semiconductor substrate 52 and the solar cell lead wire 10, the case in which sufficient bondability is not obtain is often seen even in the case of using a solar cell lead wire which conventionally has no problem of bondability. The semiconductor substrate 52 is bonded to the solar cell lead wire 10 by a formation of an intermetallic compound (e.g., Ag3Sn) between an electrode material of the front surface electrode 54 (e.g., Ag) and a bonding material of the molten solder plated layer 13 (e.g., Sn). This bonding requires that a metal atom of the solder (Sn) directly collides with a metal atom of the electrode (Ag) after an oxide film is removed from a surface of the molten solder plated layer 13 and from a surface of the front surface electrode 54 due to flux effect, and that diffusion of an Sn atom present in the solder into a lattice of another atom (Ag) is enhanced by heating. That is, when the oxide film on the surface of the molten solder plated layer 13 is very thick, the removal of the oxide film by flux is not sufficient and a soldering defect occurs.
Since the bonding between the semiconductor substrate 52 and the solar cell lead wire 10 becomes insufficient when the soldering failure occurs between the front surface electrode 54 and the solar cell lead wire 10, a module output decreases due to mechanical removal or conductivity failure.
The patent document 1 suggests a method in which 0.002 to 0.015 mass % of P is added to solder in order to suppress generation of an oxide film on the solder surface during manufacture or in use.
In the solar cell lead wire of the patent document 1, the oxide film has a thickness of about 1 to 2 μm without discoloration up to a heating temperature of 300° C., and the oxide film has a thickness of about 5 μm with slight discoloration only after reaching 350° C. On the other hand, it is described that the oxide film already has a thickness of more than 6 μm with significant discoloration at 250° C. in the prior art. Further, the patent document 1 describes that both the invention and the prior art have an oxide film with a thickness of about 1 μm in the case without heating.